the sith were originally an alien species outside the realm of the jurisdiction of the old republic. when the first dark jedi uprising occured roughly 20 millenia prior to the movies, the dark jedi were cast out, and stumbled upon the sith worlds. the sith hailed the dark jedi as gods, and so began the sith empire, where sith magic and the dark side of the force mingled to produce terrifying powers.
after the great hyperspace wars that toppled sith lord naga sadow's power base and exiled the dark lord to yavin 4, and the great sith war that ruined sith lord exar kun's bid for power, the sith tradition remained dormant until roughly 2000 years prior to the movies. the sith species had faded into extinction, so at this time the word sith took on its new meaning, that of a cult dedicated to the dark side. the new sith eventually destroyed each other in their bid for power. the 20 thousand survivors, united under sith lord kaan, established a brotherhood of darkness based on the axiom "rule by the strong". this army was driven back by the army of light under lord Hoth to the planet ruusan, on which seven titanic battles were fought. the sith lost all but two of the battles. kaan then came up with the idea of a "thought bomb", created entirely of the dark side of the force. when hoth and his army came upon the sith, kaan triggered the bomb. the implosion of the bomb sucked all of the spirits of the sith and jedi into a state of undead unrest in the valley of the jedi, only to be liberated by kyle katarn ten years after the battle of endor.
the republic thought that all of the sith were exterminated at this violent battle, but one remained: darth bane. he remade the cult on a two at a time basis: master and apprentice. thus all sith afterwards bear the title of darth.
the end of the sith seemed to come with the deaths of darth maul, darth tyranus, and darth vader. in fact, darth sidious, also known as palpatine, survived in a series of clones, the last of which was destroyed, ironically enough, by han solo, who shot him in the back with a blaster. after him there were no more sith to continue the tradition, but plenty of dark side followers remained in various guises.

marka ragnos was a sith lord of immense physical power, with a frightening grasp of the dark side of the force.

there is a power called the force that every living thing poseses like every thing there are two sides to this the light side (jedis use:luke skywalker, obi-wan, yoda ect.) then there is the dark side(siths use vader,maul,Sidious ect)the sith do not see it as evil more of power, a weapon, some 'sith'dont see the force as light or dark but as jsut the force this is frowned upon by the jedi councle (they are closed minded fools)

Supreme Chancellor: Remember back to your early teachings. "All who gain power are afraid to lose it." Even the Jedi.
Anakin Skywalker: The Jedi use their power for good.
Supreme Chancellor: Good is a point of view, Anakin. The Sith and the Jedi are similar in almost every way, including their quest for greater power.
Anakin Skywalker: The Sith rely on their passion for their strength. They think inward, only about themselves.
Supreme Chancellor: And the Jedi don't?
Anakin Skywalker: The Jedi are selfless... they only care about others.
Supreme Chancellor: Did you ever hear the tragedy of Darth Plagueis "the wise"?
Anakin Skywalker: No.
Supreme Chancellor: I thought not. It's not a story the Jedi would tell you. It's a Sith legend. Darth Plagueis was a Dark Lord of the Sith, so powerful and so wise he could use the Force to influence the midichlorians to create life... He had such a knowledge of the dark side that he could even keep the ones he cared about from dying.
Anakin Skywalker: He could actually save people from death?
Supreme Chancellor: The dark side of the Force is a pathway to many abilities some consider to be unnatural.
Anakin Skywalker: What happened to him?
Supreme Chancellor: He became so powerful... the only thing he was afraid of was losing his power, which eventually, of course, he did. Unfortunately, he taught his apprentice everything he knew, then his apprentice killed him in his sleep. It's ironic he could save others from death, but not himself.
Anakin Skywalker: Is it possible to learn this power?
Supreme Chancellor: Not from a Jedi.

The evil version of the Jedi who has embraced the Dark Side of the Force. The Sith were suppressed for one-thousand years by the Jedi Order, until the first three Star Wars movies, when Palpatine/Sideous used deception to rise to power and nearly exterminate the Jedi with the help of Darth Vader, who was once known as Anakin Skywalker.

Sith are people who follow and embrace the Sith philosophy. That philosophy involves embracing one's emotions (positive and negative) to become powerful and, eventually, free one's self (physically and metaphorically). The Sith philosophy is centered around the Sith Code:

Peace is a lie, there is only passion.
Through passion, I gain strength.
Through strength, I gain power.
Through power, I gain victory.
Through victory, my chains are broken.
The Force shall free me.

1545, from L. insithiosus "deceitful”, from insithiæ (pl.) "plot, snare, ambush,"
1 A person, usually female, whose sole purpose in her sith life is to inflict paralyzing mental anguish upon those unfortunate enough to be caught in her crazed mind web (see Vortex)
2 Operating or proceeding in an inconspicuous or seemingly harmless way but actually with grave effect: a sithlike dormant illness.
3 A girl who thrives on mind games of the most evil nature. This type of Sith does not eat, drink or sleep. She is a parasite and her host is the untouched most vulnerable part of your brain and soul. Like bacteria, this brand of Sith is capable of asexual reproduction. It creates an identical copy of itself without contribution from another source. This Sith comes in various forms, from attractive to hag-like and decrepit. Their insides are all alike, a dark chasm of unimaginable torture. Her seed may be planted with a seemingly harmless question or comment. Once the victim makes eye contact, or foolishly engages in conversation with this type of Sith, their death process rapidly increases. Victims can be identified by rapid pacing, profuse sweating, constant shaking of the head coupled with deep sighs, and eventual incoherent babbling.
4 A powerful species that cloned a female homo sapien between the ages of 18 and 55 who shepherds for the devil.
5 Of or relating to The Dark Side.

Any individual who can bring about chaos in an otherwise pleasant situation. The Sith is generally surrounded by 165 mile per hour winds and no less than 30 minions. The Sith is not to be underestimated as his/her vortex of evil energy can tear down buildings, train tracks, and engulf entire universes.

Sam: "What a beautiful day it is today."
Sara: "Not a cloud in the sky, so peaceful."
Sam: "Look at that asshole over there throwing rocks at children and pissing on that Jesus statue.
Sara: "That asshole is a real piece of SITH."